1. Field of the Invention
The present invention relates to the determination of petrophysical properties of geologic structures and, more particularly, to resistivity well logging using nuclear magnetic resonance.
2. Description of the Related Art
Measurement of rock formation resistivity by electrode well logging is an established technique for estimating the petrophysical properties of the formation surrounding a xe2x80x9cconductingxe2x80x9d borehole. Numerous devices employing various arrays of electrodes are, or have been, in use since the first resistivity log was recorded in Pechelbronn, France in 1927 by Conrad Schlumberger. All such devices have in common the production of a current density field {overscore (J)} in the formation by an electric power source and the mapping of potential differences along the borehole or the mapping of electrode potentials required to maintain a specified current distribution in the borehole. See, Bassiouni, Zaki, xe2x80x9cTheory, Measurement, and Interpretation of Well Logsxe2x80x9d; Society of Petroleum Engineers; Richardson, Texas, 1994, Chapter 5. All prior art electrode devices measure voltages or currents at the internal surface of the borehole. Subsequent analysis is then performed to loosely correlate these borehole measurements with some of the petrophysical characteristics of the surrounding formation.
Induction tools were introduced in the mid-1940""s to estimate resistivity in nonconducting boreholes. These devices magnetically induce a current flux in the formation surrounding the borehole, which formation acts as a lossy distributed mutual inductance between two or more measuring inductances. Exemplary of such a device is that shown and described in U.S. Pat. No. 5,428,293 to Sinclair et al.
While such electrode and induction tools have been widely used over the years, they have not proven to be fully satisfactory because they provide only gross approximations of resistivity distributors. Attempts have been made to overcome some of the disadvantages of both induction and of direct contact electrode current and voltage measurement devices by using other logging techniques.
Nuclear magnetic resonance devices measure other related characteristics of the rock formation surrounding a borehole. Nuclear magnetic resonance devices have been applied, for example, to measure such geophysical properties as porosity, pore size distribution, bulk fluid volume, and irreducible bound fluid volume of geological formations surrounding a borehole. Applications of this type are exemplified in U.S. Pat. No. 4,933,638 to Kenyon; U.S. Pat. No. 5,212,447 to Paltiel; U.S. Pat. No. 5,280,243 to Miller; U.S. Pat. No. 5,389,877 to Sezginer; U.S. Pat. No. 5,412,320 to Coates; U.S. Pat. No. 5,432,446 to Macinnis; U.S. Pat. No. 5,486,761 to Freedman; and U.S. Pat. No. 5,557,200 to Coates.
Representative of magnetic resonance logging tools is a device marketed under the mark MRIL by Numar Corporation. The Numar device has a sensitive volume approximating a thin cylinder 24 inches in height, 16 inches in diameter, and of one millimeter slice thickness surrounding a borehole of 8 to 12 inches diameter. This device permits measurements to be made peripheral to the borehole mud, the mudcake on the borehole wall and often to the flushed zone and the transition zone, yielding an improved estimate of the properties of the uninvaded formation surrounding the borehole relatively free of borehole effect. See, Bassiouni; op. cit. p. 71, 72.
While the value of concordant resistivity data has long been appreciated, and while the application of nuclear magnetic resonance techniques to the derivation of reasonably accurate information on pore size, bulk fluid volume and similar physical characteristics of geologic formations has been recognized, the advantages attendant to the simultaneous use of nuclear magnetic resonance for additionally determining the resistivity of geologic structures surrounding a borehole have not heretofore been realized.
Accordingly, it is an object of the present invention to provide in an embodiment a resistivity well logging method utilizing nuclear magnetic resonance.
A further object of the present invention is to provide, in an embodiment, a method for estimating resistivity in a formation surrounding a borehole utilizing magnetic resonance. The method includes providing a magnetic resonance well logging tool with a Faraday shield, and placing the magnetic resonance well logging tool and Faraday shield in the borehole and capacitively or conductively coupling the same with the formation surrounding the borehole. The method further includes energizing the formation surrounding the borehole by selectively applying phase modulating voltages to the Faraday shield and an upper and a lower coupling member to establish current fields (i) in axial and radial directions, respectively, in the formation surrounding the borehole, and/or (ii) in at least one of axial and radial directions in the formation surrounding the borehole. The method further includes measuring the current strength and distribution of the current fields by detecting phase modulation of spins of materials in the sensitive volume of the magnetic resonance tool.
It is also an object of the present invention to provide, in an embodiment, a method for determining resistivity in a formation surrounding a borehole. The method includes capacitive or conductive coupling a phase-modulating current to the formation by placing first, second, and third spaced coupling members in the borehole and selectively applying low frequency power signals to the first and second coupling members and to the second and third coupling members, respectively. The method further includes placing a well logging tool in the borehole in proximity to the second coupling member.
Another object of the present invention is to provide, in an embodiment, another method for determining resistivity in a formation surrounding a borehole. The method includes placing capacitive coupling members above and below a center capacitive coupling member containing a magnetic resonance well logging tool, and selectively establishing current fields (i) in axial and radial directions, respectively, in a sensitive region around the tool, and/or (ii) in at least one of axial and radial directions in a sensitive region around the tool. The method further includes determining resistivity in the sensitive region around the tool utilizing the current fields.
It is yet a further object of the present invention to provide, in an embodiment, yet another method for determining resistivity in a formation surrounding a borehole. The method includes placing a magnetic resonance well logging tool in the borehole and energizing the tool to produce a magnetic resonance sensitive volume thereabout. The method further includes selectively measuring current flow in the formation (i) both perpendicular to and parallel to the borehole axis within and adjacent to the sensitive volume, and/or (ii) wherein the current flow is at least one of perpendicular to and parallel to the borehole axis within and adjacent to the sensitive volume. The method further includes determining resistivity in the formation utilizing the current flow which is measured.
Another object of the present invention is to provide, in an embodiment, a further method for determining resistivity in a formation surrounding a borehole. The method includes placing a magnetic resonance well logging tool having a Faraday shield in the borehole and capacitively or conductively coupling the same with the formation surrounding the borehole. The method further includes placing upper and lower coupling members in the borehole above and below the Faraday shield of the magnetic resonance well logging tool, respectively, and capacitively or conductively coupling the same with the formation surrounding the borehole. The method further includes selectively applying voltages to the Faraday shield and the upper and lower coupling members to establish current fields (i) in axial and radial directions, respectively, in the formation, and/or (ii) in at least one of axial and radial directions in the formation. The method further includes measuring the resulting nuclear magnetic resonance signals to determine resistivity within the formation.
A further object of the present invention is to provide, in an embodiment, a method for measuring diffusion coefficient and spin relaxation time in a nuclear magnetic resonance well logging system. The method includes placing a nuclear magnetic resonance well logging device in a borehole having a surrounding formation, and energizing the nuclear magnetic resonance well logging device to generate a phase modulating current at a selected frequency and with a predetermined magnetic resonance pulse interval. The method further includes varying the intensity of the phase modulating current and its frequency and the magnetic resonance pulse interval so as to change the amplitude of a magnetic resonance signal, and determining a value for diffusion coefficient and a value for spin relaxation time utilizing the magnetic resonance signal.
The methodology according to an embodiment of the present invention of resistivity well logging may be used with any suitable well logging tool, but is particularly well suited for use with magnetic resonance tools since magnetic field gradients are produced by the application of time-varying electric fields as disclosed in applicant""s U.S. Pat. No. 5,412,322, entitled xe2x80x9cApparatus and Method for Spatially Ordered Phase Encoding and for Determining Complex Permittivity in Magnetic Resonance by Using Superimposed Time-Varying Electric Fields,xe2x80x9d which is incorporated herein by reference.
The data generated by the magnetic resonance tool in accordance with an embodiment of this invention may be obtained by using any suitable output recovery technique, but is most suitably retrieved by the implementation of techniques of demodulation and detection of phase-modulated magnetic resonance signals, as more fully disclosed herein.
In the implementation of an embodiment of the present invention, electrode arrays supplied by periodic voltage or periodic current sources are placed in either xe2x80x9cconductingxe2x80x9d or xe2x80x9cnon-conductingxe2x80x9d boreholes and are configured to produce either predominately radial or predominately axial periodic current fields. These fields are used to phase modulate signals produced by spin distributions created by a nuclear magnetic resonance well logging device. Measurements may be made in low conductivity boreholes by capacitively coupling Very Low Frequency currents (e.g. 1-10 KHz) to the conductive formation surrounding the borehole through impedance matching circuits.
A phase-modulated magnetic resonance signal consisting of a line spectrum is created by these current fields which can then be xe2x80x9cdemodulatedxe2x80x9d by convolution and cross-correlation with the master radio frequency oscillator frequency H1. Individual sideband xe2x80x9clinesxe2x80x9d can be xe2x80x9cdetectedxe2x80x9d by convolution and cross-correlation with integral multiples of the phase-modulating frequency of the current field, all of which is the radio engineering embodiment of statistically determining the amplitude of low power hidden periodicities in a stationary random process when the frequency of these periodicities is a known integral multiple of a reference signal available from a master oscillator. The central and the peripheral components of apparent formation resistivity Ra may be estimated separately. Spin-spin relaxation T2 and diffusion D may be estimated in the bulk non-surface associated large pore component of formation fluid.
The invention can also be used in measuring properties of samples in a laboratory setting, as well as in situ logging-type including logging/measuring while drilling (LWD/MWD) measurements.
Additional objects, advantages and features of embodiment of the present invention will become apparent to those skilled in the art from the following description of the preferred embodiment when taken in conjunction with the attached appendices and accompanying drawings.